Press machines are configured to manufacture a product by press working to be performed in cooperation between an upper die and a lower die while raising and lowering a slide. One important factor to determine the precision of the product is that a bottom dead center position of the slide is maintained to be constant.
In general, the bottom dead center position and a press speed have such a close relationship as illustrated in FIG. 11. In FIG. 11, the horizontal axis represents the number of shots of the press machine, and the vertical axis represents the bottom dead center position of the slide and the press speed. In FIG. 11, the bottom dead center position is as indicated by the slope part during an accelerated press operation, and by the horizontal part during a constant-speed press operation. Specifically, as the press speed increases, a downward inertial force of the slide at the bottom dead center increases, and hence the bottom dead center position of the slide becomes lower along with the increase in press speed. When the press speed is constant, the fluctuation of the bottom dead center position is significantly small.
As described above, it is important to determine whether or not the bottom dead center position is a proper position, and to set a proper bottom dead center position during the press operation.
As a determination device, there is proposed a device that performs the determination based on the number of depressed portions formed by marking (see Japanese Patent Application Laid-open No. 2010-142852). Specifically, the product is marked by punches having different heights, and the bottom dead center position is determined based on the number of marks. The determination result is used for correcting the bottom dead center position in subsequent manual work.
As a bottom dead center position correcting device, most of the devices include a sensor for detecting a die height of the press (for example, a distance from a top surface of a bolster to a bottom surface of the slide), and are configured to automatically correct an actual bottom dead center position of the slide to be adjusted to a target bottom dead center position while constantly controlling drive of a slide adjusting mechanism based on a detection signal from the sensor.
A representative improvement measure for the bottom dead center position correcting device (Japanese Patent Application Laid-open No. 2009-274080) is described. This improved device focuses on characteristics of fluctuation of the bottom dead center position during the accelerated operation and the constant-speed operation, and is configured to reduce a control load while ensuring correction accuracy in both cases. Specifically, when the press speed (SPM) changes, the acceleration of the slide changes in accordance with the SPM (as a result, the bottom dead center position changes), and hence the improved device corrects the bottom dead center position of the slide on a single-cycle basis. When the SPM is stable, on the other hand, the improved device is configured to correct the bottom dead center position of the slide on a multi-cycle basis.
By the way, as a press machine that manufactures a lamination product (completed by punching and laminating thin plates such as magnetic steel sheets), there are known a press machine of a squeeze ring holding type, which is capable of laminating punched thin plates while holding the thin plates by a squeeze ring (Japanese Patent Application Laid-open No. H07-204764), and a press machine of an actuator raising/lowering type, which laminates half-finished lamination products while lowering the half-finished lamination products by an actuator (for example, a hydraulic cylinder) (Japanese Utility Model Examined Publication No. H02-25549).
The press machine of the squeeze ring holding type includes a squeeze ring provided below a punching die, and this squeeze ring has a diameter slightly smaller than a diameter of the die. The press machine is configured to hold a punched thin plate (such as a core piece) while receiving the thin plate by the squeeze ring, and subsequently laminate the thin plate by being crimped with a thin plate that is punched out through use of a punch. In this type of press machine, there is proposed an improvement measure to increase a force for holding the thin plate by arranging magnets around the squeeze ring in preparation for further increase in diameter and thinning of the thin plate (Japanese Patent Application Laid-open No. 2005-103551).
The press machine of the actuator raising/lowering type is configured to punch a thin plate through use of a press, and form crimping projections having depressed portions on a front surface of the thin plate and projecting portions on a back surface thereof (see FIGS. 4 and 5 of the present application). In a laminating die, in a laminating process before the product is completed, a fitting and crimping pressure is applied to the half-finished products. Then, the plurality of half-finished products are laminated to obtain the product (such as a motor core and a transformer). The actuator supports the half-finished products from the bottom in the lower die so as to smoothly carry out the above-mentioned processing steps.
Specifically, the actuator is capable of lowering a workpiece support unit (receiving base 120) to a product ejecting position (see (ii) of FIG. 7), which is located lower than a press working position (see (i) FIG. 7) by a distance corresponding to a predetermined amount or more, and raising the workpiece support unit so as to be returned to the press working position (see (iii) of FIG. 7) after ejecting the product. Please see FIGS. 6 and 7.
More specifically, a lamination core having crimping projections and depressions formed therein in a previous step for each press stroke is finally punched into the laminating die 8. The actuator 100 inside the die 8 generates a receiving force when the lamination core is punched and crimped through use of the punch, and is lowered by an amount corresponding to a plate thickness of the lamination core for each crimping (see (i) of FIG. 7). Further, when ejecting the product (having the lamination cores laminated at a predetermined lamination thickness) from the die, the actuator 100 (hydraulic cylinder) is temporarily depressurized and lowered to the product ejecting position (see (ii) of FIG. 7), and after the product is ejected from the die, the actuator 100 is raised and pressurized again (see (iii) of FIG. 7).
Further, every time a predetermined number of the above-mentioned lamination cores are punched into the die, one of the lamination cores has only the depressed portions formed therein (without the projecting portions). Due to this lamination core, a lamination core that is punched into the die prior to this lamination core is not coupled to a succeeding lamination core that is punched into the die subsequently to this lamination core.
Along with the movement of the actuator, the bottom dead center of the slide of the press machine is also repeatedly lowered and raised (recovered) abruptly in accordance with a clearance inside a suspension for driving the slide and a load on the actuator. When the actuator inside the die is lowered so as to eject, from the die, the lamination cores that are laminated at a predetermined lamination thickness, the receiving force generated by the actuator against the punch when the lamination core is punched and crimped is eliminated, and hence the bottom dead center of the slide is lowered abruptly.
FIG. 12A is an explanatory timing chart illustrating a fluctuation of the bottom dead center position mainly at the time when the press speed increases according to a conventional example (the bottom dead center position of the slide is not corrected). The horizontal axis represents the number of shots of the press machine, and the vertical axis represents the bottom dead center position of the slide and the press speed. The bottom dead center position is lowered along with the increase in press speed. Further, along with the depressurization and the lowering operation of the actuator, the receiving force generated by the actuator against the punch when the lamination core is punched and crimped is eliminated, and hence the bottom dead center position is also lowered abruptly.
FIG. 12B is an explanatory timing chart illustrating a fluctuation of the bottom dead center position mainly at the time when the press speed is stable according to the conventional example (the bottom dead center position of the slide is not corrected). Along with the depressurization and the lowering operation of the actuator or the pressurization and the raising operation thereof, the bottom dead center position of the slide is lowered or raised abruptly in accordance with the clearance inside the drive mechanism for driving the slide and the load on the actuator. The lowering amount changes depending on the presence and absence of a stopper, the clearance inside the press machine, or the amount of the load on the actuator.
Note that, there are many cases where mechanical measures are taken so as to prevent trouble with the precision of the product even when the bottom dead center position is lowered. Specifically, even when the clearance inside the suspension for driving the slide is large and therefore the bottom dead center is lowered significantly, the lowering amount is restricted by a stopper block for the bottom dead center, which is provided in the die.
As described above, in the conventional example, the actuator that causes excessive disturbance on the bottom dead center position of the slide is built into the press machine, and hence the automatic correction of the bottom dead center position is not performed in most cases.
In view of the above, the inventor of the present invention has attempted to introduce the bottom dead center position correcting device into the press machine of the actuator raising/lowering type. The bottom dead center position correcting device built into the press machine is, as described above, configured to perform, based on the target bottom dead center position and a detected bottom dead center position obtained during the press operation, positional correction so that the actual bottom dead center position of the slide is adjusted to the target bottom dead center position.
FIG. 9A illustrates a change of the bottom dead center position in a case where the automatic correction is performed in every cycle shot mainly at the time when the press speed changes. The horizontal axis represents the number of shots, and the vertical axis represents the bottom dead center position of the slide and the press speed. In FIG. 9A, the region lower than the position of “0” indicates a direction in which the bottom dead center position is lowered, and the region higher than the position of “0” indicates a direction in which the bottom dead center position is raised. Further, FIG. 9B illustrates a change of the bottom dead center position in a case where the automatic correction is performed in every cycle shot at the time when the press speed is constant.
Specifically, when the actuator is depressurized and lowered or pressurized and raised during the correction control for the bottom dead center position, an abrupt and excessive change occurs in the bottom dead center position of the slide. Even when the bottom dead center is monitored in every shot, due to the feedback control, it is impossible to prevent the abrupt and excessive change of the bottom dead center position that is caused by the disturbance such as the raising and lowering operation of the actuator. This is because the bottom dead center position correcting device executes the correction in a subsequent stroke from the fact that the actual bottom dead center position deviating from the target bottom dead center position is present. In other words, irrespective of whether the press operation is performed at changing speed or constant speed, the bottom dead center position that is abruptly changed due to the disturbance inevitably results in the actual bottom dead center position deviating from the target bottom dead center position. As illustrated in FIGS. 9A and 9B, there occur abrupt and excessive fluctuations Dyu and Dyd of the bottom dead center position corresponding to a delay in response.
Next, in a case where the bottom dead center position is corrected through averaging performed on a multi-cycle basis (FIG. 10), an average value of a plurality of arbitrary shots including current and previous shots serves as a correction value, and hence the delay occurs. The operation is brought into an oscillatory state due to the abrupt and excessive change of the bottom dead center position that occurs unexpectedly. The oscillation occurs in addition to the delay. That is, the operation is brought into an uncontrollable state, which increases a risk in that the bottom dead center position fluctuates in a severely undulating manner.
By the way, in the case of the lowering of the bottom dead center position due to the lowering operation of the actuator (“Dyd” of FIGS. 9A and 9B), the lowering amount may be restricted by introducing the auxiliary mechanical stopper block as described above, and hence there may arise no particular problem in the manufacture of the product. On the other hand, it is in some cases preferred to avoid suspending the correction of the bottom dead center position under a state in which the stopper block is not introduced.
Even during the correction control for the bottom dead center position, however, the actual bottom dead center position is raised in an abrupt and excessive manner (“Dyu” of FIGS. 9A and 9B) due to the raising operation of the actuator. Then, in the step (the crimping projections and depressions are formed) prior to the step in which the lamination core is punched into the die, the depth of the crimping projections and depressions becomes smaller than originally intended. As a result, the fitting and crimping pressure decreases, and hence the crimping is insufficient, which causes such an adverse effect that the thin plates are separated from each other. This adverse effect is caused from the fact that, in the case of the raising operation, the mechanical restricting unit such as the stopper block cannot be provided unlike the case of the lowering operation.
Further, when the bottom dead center position fluctuates significantly toward the lower side due to the oscillation (FIG. 10), the depth of the projections and depressions becomes larger than originally intended, and hence the projecting portions are disengaged due to crimping failure. In addition, when the stopper block is provided in the die, the amount of abutment of the slide against the stopper block increases considerably, and hence the slide bounces significantly. As a result, the precision of the product is deteriorated and the life of the die is reduced. Eventually, the risk of an overload on the press machine increases.
As described above, in the conventional press machine, a considerable amount of raising and lowering shift of the actuator is added as the disturbance, and hence, to avoid the adverse effect induced by this disturbance, the bottom dead center position correcting device has not presumably been introduced in most cases. Further, even in the bottom dead center position correcting device (Japanese Patent Application Laid-open No. 2009-274080), the abrupt and excessive change of the bottom dead center of the slide has not been taken into consideration.